It seems evident that the removal of the underdeveloped and loose lamina does not increase the so-called "instability" of the lumbosacral junction. If the defect in the pars interarticularis is supposed to cause instability, then the removal of the mobile fragment should not increase the instability. Surely, no surgeon who has examined the loose fragment could believe that it has any function in holding the fifth lumbar vertebra to the sacrum.

Our concept of the pathomechanics of this condition would answer all that is not explained by the prevalent concept of instability at the lumbosacral junction. If the loose lamina is not very mobile, there may be no irritation of the dura. If the defect in the pars interarticularis is more posteriorly situated, pressure on the fifth lumbar root could not develop. Conversely, if the defect were anterior, the likelihood of compression of this root would be greater. This pathological picture also explains why patients go for years without symptoms, since considerable time may be necessary for the growth of a fibrocartilaginous mass large enough to compress and irritate the fifth lumbar root. It would explain, moreover, the sudden development of symptoms following a flexion injury, since the fibro-cartilaginous mass would pull upon the adhesions between it and the nerve roots. Of even more importance is the explanation of the susceptibility of these patients to hyperextension injury. The jamming down of the loose lamina causes local irritation of the dura, stretching of the first sacral roots, and a more downward pressure upon the fifth lumbar roots which are already in a precarious condition.

Coccygeal pain may be explained as the result of adhesions between the ligamentum flavum, the inferior edge of the loose lamina, and the dura. Tugging or pulling on these adhesions may give rise to pain in the lumbosacral area and, in many instances, to coccygodynia. That coccygeal pain may result from adhesions about the lower dural sac cannot be doubted, since we have relieved these symptoms by division of the adhesions to the dural sac.

The successful results of the conventional fusion operation may be explained on the basis of the prevention of dural and nerve-root compression and irritation from the mobile lamina, and perhaps by shrinkage of the pseudarthrosis, or of the fibrocartilaginous mass following fixation. Similarly, fusion may give relief in those few cases in which symptoms are arising from protrusion of the fourth lumbar disc.

We do not advise the decompression procedure in children because the rare instances of forward displacement have occurred in this group. We have recently seen a patient, aged eight years, who is already having radicular symptoms, with definite compression of the fifth lumbar root. In this type of case the simple fusion operation would not be likely to give relief. We feel that conservative treatment, consisting in postural exercises and support in the flexed position, should be used until the late teens when growth is completed.

In adults the advantages of the decompression operation are obvious. In addition, many patients who have been unable to afford the long period of disability necessitated by the conventional fusion operation will be able to accept relief.

In conclusion, the authors present the thesis that the majority of patients with symptomatic spondylolisthesis show radicular symptoms in addition to the local ache and tenderness in the lumbosacral area. Careful neurological examination of most of these patients reveals involvement of the fifth lumbar and, in some cases, of the first sacral root. Surgical exploration has consistently revealed nerve-root compression by a mass of fibrocartilaginous tissue at the defect in the pars interarticularis of the fifth lumbar vertebra. Movement of the loose lamina may, in addition, cause nerve-root and dural irritation and compression. Surgical decompression by removal of the loose lamina and the mass of fibrocartilaginous tissue at the defect in the pars interarticularis has given relief, not only of the radicular symptoms, but also of the low-back pain.